Ag 3 (SO 3 F) 4 : A Rare Example of a Mixed‐Valent Ag II /Ag I Compound Showing 1D Antiferromagnetism

Dark brown Ag I 2 Ag II (SO 3 F) 4 , known for over 30 years, is the first known example of four well‐characterized mixed‐valent (1+/2+) compounds of silver. It crystallizes in the monoclinic space group, P 2 1 / c , with a = 5.33670(19) Å, b = 12.9486(4) Å, c = 19.5976(7) Å, β = 100.6407(13)°, V = 1330.95(8) Å 3 , Z = 4 and d calcd. = 3.59 g cm –3 . Its chemical formula is best written as [Ag I 2 (SO 3 F)] + [Ag II (SO 3 F) 3 ] – . Ag II centres form 1D chains linked through OSO bridges that result in pronounced antiferromagnetic coupling with T N = 225 K and a superexchange coupling constant ( J ) of –7.5 meV per pair of coupled Ag II cations. Ligand environments around Ag I and Ag II differ substantially, which suggests a genuine mixed‐ (i.e., localized) and not intermediate‐valent (i.e., delocalized) character of the title compound. Indeed, electronic absorption is not observed up to 7500 cm –1 , so the intervalence charge‐transfer transition across the electronic band gap must fall above 0.8 eV. The compound is stable up to a mere 75 °C, which marks the onset of its thermal decomposition to Ag I SO 3 F and the SO 3 F · radical. Ag I 2 Ag II (SO 3 F) 4 is, after AgSO 4 , the second‐known 1D antiferromagnetic semiconducting oxa derivative of Ag II .